1. Field of the Invention
The present invention relates to an organometallic complex and an organic electroluminescent device employing the same, more particularly relates to an organometallic complex having excellent luminescence properties in the range of the blue wavelengths.
2. Description of the Related Art
An organic electroluminescent display (organic EL display) is an active light-emitting display using a fluorescent or a phosphorescent organic compound that emits light in response to the recombination of holes injected from an anode and electrons injected from a cathode in the organic film when an electric current is applied to a thin film (hereinafter referred to as “organic film”) composed of the fluorescent or phosphorescent organic compound. Further, since an organic electroluminescent display has various advantages such as light weight, simple structure which can be manufactured in an uncomplicated manufacturing process, a large viewing angle, high brightness, full color, low power consumption, low driving voltage and complete expression of moving picture, the display is very useful in portable electronic devices.
Luminescence of an electroluminescent display is obtained by injecting holes from an anode and electrons from a cathode into a light-emitting layer. The holes and the electrons are recombined in the light emitting layer to form excitons, and emit light with a wavelength corresponding to a band gap when the exciton radiatively decays.
The light emitting layer-forming materials are classified into fluorescent materials using single state excitons and phosphorescent materials using triple state excitons, according to a luminescence mechanism.
The fluorescent material using singlet excitons has been employed in conventional organic electroluminescent devices. In this case, however, three-fourth of the energy produced by the excitons is not used.
When the fluorescent materials are used as luminescent materials, that is, a luminescence process mediated by an exciton in a singlet state is used, the internal quantum efficiency is at most about 25%. Furthermore, the actual external quantum efficiency is at most 5% since an extraction efficiency of light is affected by the refractive index of substrate materials. There are such limitations as long as fluorescence of singlet state excitons is used. Thus, there have been various attempts to increase luminescence efficiency by using 75% of energy produced by triplet state excitons produced by recombination.
The transition from a triplet state to a singlet state is a forbidden transition, is non-luminescent, and is hard to use.
However, when heavy metals such as Ir, Pt, Rh, and Pd are included in luminescent materials, excitons can transit from a triplet state to a singlet state due to a property rendered by a spin-orbital coupling.
By using tris-orthometalated complex of Iridium (III), Ir(ppy)3, in which three 2-phenylpyridine ligands are coordinated to Ir(III), as a light emitting layer-forming material, the external quantum efficiency may increase up to 8%, which is higher than the maximum external quantum efficiency, that is, at most 5%, of standard fluorescent materials, and this was reported in 1999 (Applied Physics Letters, Vol. 75, P.4 (1999)). However, the application of such materials to actual displays is restrictive since these materials are limited to green luminescence. Thus, it is necessary to develop phosphorescent materials, which emit light in other color ranges.
The complexes in which an aromatic based compound consisting of a carboxylic acid group as a binding site with iridium was introduced as a ligand, are used as phosphorescent materials for organic electroluminescent devices. A particular example of such a compound includes a compound in which trifluoroacetate is introduced into a 2-phenylpyridine based Ir(III) compound (U.S. Pat. Appl. No. 2002/0048689 A1).